Create bg_id and wthh_id endogenously.

[MImmesberger]

What problem do you want to solve?

Create the bg_id and wthh_id for households with exactly 1 fg endogenously.

Based on the discussion in #763

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 30.76923% with 9 lines in your changes missing coverage. Please review.

Project coverage is 89.41%. Comparing base (d41cc45) to head (53c7b2f).

:exclamation: Current head 53c7b2f differs from pull request most recent head ca89d68

Please upload reports for the commit ca89d68 to get more accurate results.

Files	Patch %	Lines
...ttsim/transfers/arbeitsl_geld_2/arbeitsl_geld_2.py	20.00%	4 Missing :warning:
...gettsim/transfers/benefit_checks/benefit_checks.py	33.33%	4 Missing :warning:
.../transfers/arbeitsl_geld_2/arbeitsl_geld_2_eink.py	50.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #768 +/- ## ========================================== - Coverage 89.62% 89.41% -0.21% ========================================== Files 52 52 Lines 3768 3780 +12 ========================================== + Hits 3377 3380 +3 - Misses 391 400 +9 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[MImmesberger]

Improved version in #778